Many circuits require automatic tuning capability in order to be practicable. One way to accomplish automatic circuit tuning is to use one or more variable capacitors. In low power circuits, for example, a varactor, which comprises a semiconductor device having a junction capacitance that varies with bias voltage, is often used for automatic tuning. Alternatively, for both low and high power applications, mechanical actuators (e.g. stepper motors) with feedback may be used to vary capacitance. The latter solution, however, is generally too costly and hence not practical for many applications.
In electrodeless high intensity discharge (HID) lamp ballasts, for example, automatic tuning is desirable because the output impedance of the ballast changes when a transition is made between starting and running conditions. In an electrodeless HID lamp, an arc discharge is generated by establishing a solenoidal electric field in a gas contained within an arc tube. In particular, the solenoidal electric field is created by the time-varying magnetic field of an excitation coil which is disposed about the arc tube. To maximize efficiency of an HID lamp, the degree of coil coupling between the magnetic field and the arc discharge must be maximized. Since the degree of coupling increases with frequency, electronic ballasts used to drive HID lamps operate at high frequencies in the range from 0.1 to 30 MHz, exemplary operating frequencies being 13.56 and 6.78 MHz. These exemplary frequencies are within the industrial, scientific, and medical (ISM) band of the electromagnetic spectrum in which moderate amounts of electromagnetic radiation are permissible; and such radiation is generally emitted by an electrodeless HID lamp system.
Operation of an HID lamp ballast at the series resonant frequency of the load circuit maximizes power output. However, operation at a frequency slightly higher than the series resonant frequency of the load circuit maximizes ballast efficiency. Hence, for maximum efficiency, operation is slightly "off" resonance, and a specific ballast load resistance and phase angle are required. To this end, the impedance of the ballast load, including that of the arc discharge as reflected into the ballast load, must be matched to the required ballast load resistance and phase angle. As described in commonly assigned, copending U.S. Pat. application Ser. No. 472,144, of J.C. Borowiec and S.A. El-Hamamsy, filed Jan. 30, 1990, which is incorporated by reference herein, a capacitance connected in parallel with the excitation coil is needed to match the resistive component of the ballast load impedance, and a capacitance connected in series with the excitation coil is needed to obtain the proper phase angle. However, although the series and parallel tuning capacitances provide a matched impedance under lamp-operating, or running, conditions, the output impedance of the ballast load circuit is different under starting conditions. Furthermore, in order to ensure that enough power is provided to start the lamp, the ballast should be tuned under starting conditions. Thereafter, i.e. after the lamp has started, the ballast must be tuned under running conditions for maximum efficiency operation. Therefore, it is desirable to provide a means for automatically tuning the ballast load circuit as the output impedance thereof changes between starting and running conditions.